In order to control a DC motor, a rotation speed detector, e.g. an encoder, is provided for the DC motor, which develops a signal to be applied to a microcomputer, which, in turn, provides various controls for the motor, using the signal from the encoder. In particular, for braking of the DC motor, direct-current braking or plugging is employed.
However, DC braking and plugging systems provide only a small braking force or a small brake holding force. Another problem is that the encoder mounted on the motor makes the motor large-sized, which causes increase in cost. Further, an encoder has high resolution, so delicate control can be provided only when a microcomputer is used with it. If logic circuitry is employed to carry out delicate control in response to the resolution of the encoder, a large quantity of logic circuitry is required.
An object of the present invention is to provide a control apparatus for a DC motor, which can hold or maintain a large braking force. Another object of the present invention is to provide a DC motor control apparatus which can provide delicate control of a DC motor without resort to the use of a microcomputer.
A DC motor control apparatus according to the present invention includes a rotation speed detector, which generates a pulse each time a DC motor rotates a predetermined angle. One or more rotation speed detector may be used. A servo command circuit outputs a servo command signal when it judges, based on the pulses generated by the rotation speed detector, that the speed of the motor has decreased below a predetermined value due to a stop command given to the motor. When the servo command signal is generated, a rotation direction indicating circuit outputs a rotation direction indicating signal to cause the motor to rotate in an opposite direction, in accordance with the pulses. A braking force indicating circuit generates and holds a braking force indicating signal based on the actual rotation speed at which the motor rotates after the servo command signal has been generated. A driver causes the motor to rotate in the direction indicated by the rotation direction indicating signal, while applying to the motor a braking force based on the braking force indicating signal.
When the motor speed becomes lower than the predetermined speed due to the application of the stop command, the servo command signal is generated. In response to the servo command signal, the rotation direction indicating circuit commands the motor to rotate in the direction opposite to the current direction of rotation. Also, in response to the servo command signal, the braking force indicating circuit determines the braking force in accordance with the actual rotation speed of the motor. The braking force indicating signal generated by the braking force indicating circuit is applied to the driver, which, in turn, gives the motor the determined braking force in the determined direction. The braking force indicating signal is held so that the motor can be held being braked.
The servo command circuit may include a counter which count a clock signal from a predetermined initial count each time the pulse is applied to the servo command circuit, and a servo signal generator circuit which generates the servo command signal when the count in the counter reach a predetermined count.
When the motor is rotating at a rate or speed faster than the speed corresponding to the predetermined count, the pulse is generated before the counter counts the predetermined count and the count returns to the initial count. The counter, then, resumes counting from the initial count.
When the motor is rotating at a speed slower than the speed corresponding to the predetermined value, the pulse is generated by the rotation speed detector after the counter counts the predetermined value. Accordingly, by causing the servo command signal to be generated when the counter counts the predetermined value, the servo command can be given when the motor speed decreases below the predetermined speed.
The rotation speed detector may include a first rotation speed detector circuit which generates first pulses, and a second rotation speed detector circuit which generates second pulses. When the motor is rotating in the forward direction, respective first pulses precede corresponding second pulses, and when the motor is rotating in the reverse direction, the second pulses precede the corresponding first pulses. The rotation direction indicating circuit receives the first and second pulses, and generates, as the rotation direction indicating signal, a reverse direction indicating signal when the first pulses precede the second pulses and a forward direction indicating signal when the second pulses precede the first pulses.
With the above-described arrangement, the rotation direction indicating signal corresponding to the order in which the first and second pulses occur is generated.
The braking force indicating circuit may be arranged to count a clock signal prepared from the pulses provided by the rotation speed detector, and command the driver to cause the motor to rotate with a first braking force being applied to it when the count counted is within a predetermined first range, and command the driver to cause the motor to rotate with a second braking force, larger than the first braking force, being applied to it when the count counted is within a second range of counts above the first range.
With this arrangement, if the motor does not stop even with the first braking force applied to it, so that the count counted by the counter is within the second range, the second braking force larger than the first braking force is applied to the motor so that the motor can be stopped without fail.
The rotation speed detector may be provided by a Hall effect device. When a Hall effect device is used, the servo command circuit, the rotation direction indicating circuit, and the braking force indicating circuit are implemented by logic circuits.
A rotation speed detector for controlling some types of motors is not required to have a high resolution. In such a case, the cost of manufacturing the control apparatus can be reduced by using a Hall effect device and logic circuits with the rotation speed detector.